The present invention relates generally to digital audio broadcasting (DAB) and other types of digital communication systems, and more particularly, to frequency offset estimation techniques for such digital audio broadcasting (DAB) and other types of digital communication systems.
Proposed systems for providing digital audio broadcasting (DAB) in the FM radio band are expected to provide near CD-quality audio, data services and more robust coverage than existing analog FM transmissions. However, until such time as a transition to all-digital DAB can be achieved, many broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid in-band on-channel (HIBOC) DAB systems, and are being developed for both the FM and AM radio bands.
In order to prevent significant distortion in conventional analog FM receivers, the digital signal in a typical FM HIBOC DAB system is, for example, transmitted in two side bands, one on either side of the analog FM host signal, using orthogonal frequency division multiplexing (OFDM) sub-carriers. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel.
In the United States, the frequency plan established by current FCC regulations separates each transmitting station in a geographical area by 800 KHz. Any transmitting stations in adjacent geographical areas, however, are separated from a local transmitting station by only 200 KHz. Thus, a particularly significant source of interference in such a system is known as first adjacent analog FM interference. This interference results when a portion of an FM host carrier in an adjacent geographic area overlaps in frequency with a portion of a digital signal side band. Although first adjacent analog FM interference, when present, typically affects only one of the two digital side bands, it nonetheless represents a limiting factor on the performance of DAB systems. The presence of a strong first adjacent interference signal will significantly degrade the performance of the digital signal transmissions, even when one of the two side bands is free from interference.
Frequency offset estimation techniques are implemented in most communication systems. In most digital communication systems, a frequency error is calculated using information in the time domain, and feedback or forward error correction is provided to correct the error. Time domain operation in the IBOC case, however, is significantly impacted by in-band power from adjacent channels due to first adjacent interference. In addition, timing errors in the time domain operation are coupled to the frequency error calculation.
Most conventional frequency offset estimation algorithms estimate the coarse frequency offset in the frequency domain after initially performing a partial frequency offset estimation and compensation in the time domain. Generally, the partial frequency offset estimation is performed by estimating the phase rotation of the cyclic prefix portion of the OFDM frame in the time domain. Thereafter, the corresponding partial frequency offset is readily extracted from the estimated phase rotation. For a more detailed discussion of conventional frequency offset estimation algorithms, see, for example, J. Van de Beek, xe2x80x9cTime and Frequency Offset Estimation in OFDM Systems Employing Pulse Shaping,xe2x80x9d I.E.E.E. ICUPC Conference (April, 1997), incorporated by reference herein.
A need therefore exists for a frequency offset estimation technique that provides reliable performance, even in presence of first adjacent interference. A further need exists for a method and apparatus that independently performs frequency offset estimation and frame synchronization in the frequency domain.
Generally, a method and apparatus are disclosed for frequency offset estimation in a hybrid in-band on-channel (HIBOC) digital audio broadcasting (DAB) system. The frequency offset estimation algorithm first determines the coarse frequency offset, in terms of integer number of OFDM bin separations between an actual and measured location of a correlated peak, followed by estimation and tracking of the partial (residual) offset in a continuous fashion.
Since the coarse frequency offset estimation is performed before the partial frequency offset estimation is established, the coarse frequency offset can be an arbitrary number. If the frequency offset happens to be close to a bin, the correlated output will provide a relatively clean peak. If, however, the frequency offset is in the middle of two bins, the correlated output will provide the worst-case peak. According to one feature of the present invention, a frame is correlated at a first frequency, and then the frequency is shifted by a predefined amount, such as half of the inter-bin frequency amount, xcex94f, before correlating again. The measurement with the highest peak of the plurality of frequency values is utilized. Since the correlation is performed for at least two frequencies, with a relative offset of half of the inter-bin frequency amount, xcex94f, at least one frequency will position the correlated peak near a bin.
The coarse frequency offset estimation algorithm utilizes signature sequences, such as Barker codes, to provide reference information contained in discrete known frequency points in the frequency domain, among the sub-carriers in an OFDM system. Correlation is applied in the frequency domain to identify peaks in the transmitted signal and determine the required coarse frequency offset adjustment. The calculated coarse frequency offset is applied to a forward correction mechanism and the coarse offset, in terms of an integer number of OFDM sub-carriers, is corrected.
To estimate and compensate for the partial frequency offset in the range of +ffr/2 and xe2x88x92ffr/2 (where ffr denotes the OFDM bin separation), the coarse frequency offset and compensation must have been already completed. The partial frequency offset estimation algorithm utilizes phase information contained in reference frequency points in the frequency domain. The phase rotation of the reference vector is proportional to the frequency error, and the frequency error is extracted and filtered in the time domain. The initial partial frequency offset estimation is corrected and used for continuous frequency tracking. The calculated partial frequency offset is applied to a forward correction mechanism and the partial offset, in terms of an amount less than the sub-carrier spacing, is corrected.
The inner-most bins of each upper and lower side band in an OFDM system are unmodulated. Following the course frequency offset estimation, the unmodulated bins, and all bins, are within at least half the of the OFDM bin separation. The unmodulated bins can be used to estimate the partial fraction of the frequency offset. In the presence of a frequency offset, the complex bins start rotating. The rate of the rotation is a function of the extent of the frequency offset. The partial frequency offset estimation algorithm attempts to make the rate of rotation equal to zero. The change in phase from one frame to the next is proportional to the rate of rotation, and the sign of the rotation indicates the direction of the shift.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.